Integrated circuit switching speeds are enhanced by forming transistors in silicon-on-insulator (SOI) structures. An SOI structure consists of crystalline silicon islands on an insulator layer of a bulk silicon substrate, transistors being formed in or on the individual islands. Device speed is enhanced because the electrical isolation of each island from the bulk substrate and from other islands reduces capacitive loading of the transistors from the bulk and from devices in the other islands. The problem is that it is difficult to provide a high quality crystalline semiconductor silicon layer over an insulator layer because formation of an underlying insulator layer typically introduces defects into the overlying semiconductor silicon layer. Such defects may render it impossible to form working transistors in the overlying semiconductor layer. One way of avoiding such problems is to use ion beam implantation to implant oxygen atoms well below the bulk silicon surface of a silicon wafer, leaving a surface layer of pure silicon over an underlying layer (or “box”) of silicon containing implanted oxygen atoms. The crystalline structure of the silicon surface layer may be preserved despite the transmission of the ion beam therethrough by maintaining the wafer temperature at about 600 degrees C. during the oxygen ion implantation process. Thereafter, the wafer is annealed in order to cause substitution of the implanted oxygen atoms into the crystal structure of the “box” to form a silicon dioxide insulator layer. The energy of the oxygen ion beam is selected to place the silicon dioxide box at the desired depth below the substrate surface, this depth determining the thickness of the crystalline silicon surface layer. The thickness of the silicon surface layer may be enhanced by epitaxial growth of crystalline silicon, using chemical vapor deposition, for example. While this process is precise and produces a high quality crystalline silicon surface layer, it is prohibitively expensive, because the oxygen beam ion implantation process requires many hours for a single silicon wafer, which is unacceptable.
One way around this difficulty is a layer transfer technique, in which a silicon dioxide insulator layer is efficiently formed on the surface of a silicon wafer using a standard furnace oxidation process. The overlying pure semiconductor layer is then provided by bonding a thin silicon wafer to the silicon dioxide surface of the bulk wafer. While this technique is more cost efficient, it produces a lower quality semiconductor layer. This is because the thin silicon wafer bonded to the bulk layer is formed by slicing a standard silicon wafer to produce the thin wafer. The slicing process produces a surface with a relatively high defect density. The slicing may be accomplished by implanting hydrogen (which is a relatively fast process due to the low atomic mass of the hydrogen) to form a defect layer at the desired depth. The hydrogen implanted wafer is then sliced along the defect layer to form a thin silicon wafer. The bulk wafer and the thin wafer are bonded back-to-back, leaving the sliced surface of the thin wafer as the surface in which devices (transistors) are to be formed. Defects in the sliced surface can be reduced by annealing, but some defects will remain, thus compromising device quality in the final product.
In view of the foregoing difficulties of the layer transfer technique, attempts have been made to revert to the ion implantation technique discussed above for forming the underlying silicon dioxide insulator layer, and overcoming the inefficiency (slowness) of the oxygen ion beam implantation process by implanting oxygen using plasma immersion ion implantation. Plasma immersion ion implantation is much faster than ion beam implantation because it implants ions over the entire wafer surface, in contrast to the narrow beam of an ion beam implant process which much be raster scanned over the wafer surface at a relatively slow rate. Moreover, the ion flux of the plasma immersion ion implantation process may be increased as desired by increasing the plasma source power. One difficulty with the plasma immersion ion implantation process is that it is incapable of producing sufficient ion energies (implantation depth) to avoid implanting oxygen atoms in the wafer surface layer. The silicon surface layer must be fairly free of defects, such as oxygen atoms, in order to be susceptible of epitaxial growth of additional crystalline silicon on the silicon surface layer. Specifically, the oxygen concentration in the silicon surface layer should not exceed a certain threshold, typically about 1018 cm−3 in certain cases. This is because defects in the silicon surface layer are replicated in any epitaxial silicon layer that is deposited thereover, and such defects may render the epitaxial silicon layer an amorphous or polycrystalline non-semiconductor, rather than a crystalline semiconductor. Typical plasma immersion ion implantation reactors cannot produce oxygen ion energies sufficient to minimize implanted oxygen in the silicon surface layer below the threshold for epitaxial semiconductor silicon growth. Ion energy is increased by increasing the plasma bias power applied to the wafer support pedestal of the plasma immersion ion implantation reactor. However, it has been found that plasma bias power cannot be sufficiently increased to avoid implanting oxygen in the silicon surface layer without causing arcing in the vicinity of the wafer support pedestal, and a consequently loss of control over the plasma process.
Another problem with the use plasma immersion ion implantation to form SOI structures arises in the preparation of the silicon surface layer for the epitaxial silicon deposition step. Prior to the step of epitaxial silicon deposition, the surface must be cleaned to remove impurities, such as a native oxide, for example. The problem is that plasma used during the oxygen ion implantation and/or plasma used during the surface cleaning step tends to remove material from the reactor chamber interior surfaces, creating contamination (e.g., aluminum) that is deposited on the silicon surface prior to silicon epitaxial deposition. Such contamination creates defects, which can render the epitaxially deposited silicon unsuitable.
What is needed is a way of performing plasma immersion oxygen ion implantation so as to leave an SOI silicon surface layer that is relatively free of oxygen atoms without increasing plasma bias power. What is also needed is a plasma reactor that does not contaminate the silicon layer through plasma interaction with chamber interior surfaces.